The present invention relates to a method for fabricating an image sensor, and more particularly, to a method for forming a metal interconnection in an image sensor.
An image sensor is a device for converting one or two- or higher-dimensional optical image into an electrical signal. The image sensor is mainly classified into a camera tube and a solid-state image sensor. The camera tube has been widely used in layer, control, recognition, etc, employing an image processing technology on the basis of a television, and its application technology has been developed. As the solid-state image sensor, which comes into a market currently, there are metal oxide semiconductor (MOS) image sensors and charge coupled devices (CCDs).
The CMOS image sensor converts an optical image into an electrical signal using a CMOS fabrication technology, and employs a switch mode to detect outputs one by one using MOS transistors which are made as many as the number of pixels. In particular, the CMOS image sensor has advantages in that a driving mode is simple, various scanning modes can be embodied, and a signal processing circuit can be integrated into a single chip, which can miniaturize a chip. In addition, the CMOS image sensor is inexpensive and consumes a low power, because of utilizing a compatible CMOS technique.
Recently, to reduce a noise, various attempts have been made to reduce an interlayer thickness between metal interconnections in fabricating an image sensor. One of them is to form a copper (Cu) interconnection using damascene process. The reason is that the copper has excellent interconnection characteristic in spite of relatively small thickness because it has a lower electrical conductivity than aluminum (Al).
In the method for fabricating the image sensor using the copper interconnection, an annealing process is additionally performed using a forming gas for reducing a dark current in a device after a padding process. Generally, the dark current characteristic can be reduced if the annealing process is performed at a high temperature of 400° C. or higher. This characteristic can be understood from Fick's diffusion equation as below.C(x,t)=erfc(x/(4Dt)1/2)D=D0*exp(−Ea/kT)  [Diffusion Equation]where D, t, C, Ea denote diffusion coefficient, time, concentration, and activation energy, respectively.
However, it may be difficult to perform the annealing process at a high temperature of 450° C. or higher due to a thermal degradation of the copper interconnection.